JP2024112785A - System for manufacturing high-quality semiconductor single crystal and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide systems and methods for growing bulk semiconductor single crystals, more specifically, for growing bulk semiconductor single crystals, such as silicon carbide, based on physical vapor transport.

SOLUTION: A sublimation system for growing at least one single crystal of a semiconductor material by a sublimation growing process comprises a crucible (102) having a longitudinal axis (120) and comprising fixing means for at least one seed crystal (110) and at least one source material compartment (104) for storing a source material (108); a heating system formed to generate an irregular temperature field around the circumference of the crucible at one or more defined heights along the longitudinal axis of the crucible; and a rotary drive that is operable to cause a rotational movement of the fixing means around the longitudinal axis relative to the heating system.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to systems and methods for growing bulk semiconductor single crystals, and more particularly, for growing single crystals of bulk semiconductors such as silicon carbide based on physical vapor transport.

Silicon carbide (SiC) is widely used as a semiconductor substrate material for electronic components in a wide range of applications, such as power electronics, radio frequency, and light emitting semiconductor components.

Physical vapor transport (PVT) is commonly used to grow bulk SiC single crystals, especially for commercial purposes. SiC substrates are produced by cutting slices from a bulk SiC crystal (e.g., using a wire saw) and finishing the slice surface in a series of polishing steps. The finished SiC substrate is used in the manufacture of semiconductor components, such as in an epitaxial process, where a thin single crystalline layer of a suitable semiconductor material (e.g., SiC, GaN) is deposited on the SiC substrate. The properties of the deposited monolayer and the components produced therefrom depend crucially on the quality and homogeneity of the underlying substrate. For this reason, the excellent physical, chemical, electrical and optical properties of SiC make it a preferred semiconductor substrate material for power device applications.

PVT is essentially a crystal growth method involving the sublimation of a suitable material and its subsequent resolidification onto a seed crystal, where the formation of a single crystal occurs. The source material and the seed crystal are placed inside a growth structure, and the source material is sublimated by heating. The sublimated vapor then diffuses in a controlled manner due to a gradient temperature field set up between the source material and the seed crystal, depositing on the seed and growing as a single crystal.

Conventional PVT-based growth systems generally utilize either induction or resistance heating systems to sublimate the raw material. In both cases, the core of a PVT-based growth system is the so-called reactor. Basically, a growth structure, comprising a crucible and a fixing means for the seed crystal, traditionally made from graphite and carbon materials for insulation, is placed inside the reactor and heated either by an induction coil located outside the reactor or by a resistance heater located outside or inside the reactor. The temperature in the growth structure is measured by one or more pyrometers or one or more thermocouples, installed near the overture of the growth structure. The vacuum-sealed reactor is evacuated by one or more vacuum pumps and receives a supply of inert or doping gases via one or more gas supplies to create a controlled gas (gas mixture atmosphere). All process parameters (pressure, temperature, gas flow rates, etc.) can be regulated, controlled, and stored by a computer operated system controller, which communicates with all relevant components (e.g., inverters, pyrometers, vacuum control valves, mass flow controls (MFCs), and pressure gauges).

In an inductively heated PVT system, the reactor typically includes one or more glass tubes, optionally cooled with water, with flanges at both ends to seal the interior of the reactor from the atmosphere. An example of such an inductively heated PVT system is described in the patent U.S. Pat. No. 8,865,324. FIG. 8 shows such a conventional inductively heated PVT system 800.

The growth mechanism 800 comprises a growth crucible 802, which includes a SiC supply region 804 and a crystal growth region 806. Powdered SiC raw material 808 is injected into the SiC supply region 804 of the growth crucible 802 as pre-processed starting material before the start of the growth process, e.g., placed in the SiC supply region 804. A seed crystal 810 is provided in the crystal growth region 806 on an inner wall of the growth crucible 802 facing the SiC supply region 804, e.g., on the crucible lid 812. The bulk SiC single crystal to be grown is grown on the seed crystal 810 by deposition from a SiC growth vapor phase formed in the crystal growth region 806. The bulk SiC single crystal to be grown and the seed crystal 810 may have approximately the same diameter.

The growth crucible 802, including the crucible lid 812, may be manufactured from an electrically and thermally conductive graphite crucible material. Thermal insulation (not shown in the figures) is disposed around it, which may include, for example, a cellular graphite insulation material, the porosity of which is in particular higher than the porosity of the graphite crucible material.

The thermally insulated growth crucible 802 is placed inside a tubular vessel 814, which may be configured as a quartz glass tube, forming an autoclave or reactor. An inductive heating device in the form of a heating coil 816 is arranged around the vessel 814 to heat the growth crucible 802. The growth crucible 802 is heated by the heating coil 816 to a growth temperature of more than 2000°C, in particular to about 2200°C. The heating coil 816 inductively couples an electric current to the conductive crucible wall (so-called susceptor) of the growth crucible 802. This current flows as a circulating current in the circumferential direction in the substantially circular, hollow, cylindrical crucible wall, heating the growth crucible 802 in the process. The susceptor may be made of graphite, TaC, WC, Ta, W, or other refractory metals. The primary purpose of the susceptor is to provide a heat source inside the crucible 802. When the susceptor is heated by induction, the surface of the susceptor reaches a high temperature, which is then transferred to the inside of the crucible 802 through conduction and/or radiation.

As mentioned above, the induction coil 816 is mounted on the outside of the glass tube 814 and is typically surrounded by a Faraday cage (not visible in the drawings) that provides an electromagnetic shield to block electromagnetic radiation. The induction coil 816 is wound with equidistant turns, each turn being a distance d_1 from the next turn.

Furthermore, in conventional resistively heated PVT systems, the heating resistive element is mounted inside the reactor. If the reactor is metallic, it can be cooled by water or air. Examples of resistively heated PVT systems are described in published patent applications U.S. Patent Application Publication No. 2016/0138185 and U.S. Patent Application Publication No. 2017/0321345.

Currently, these and other conventional PVT growth systems with similar components to those mentioned above are based on the idea of heating the inside of the crucible with a temperature field that is as homogeneous as possible in the radial direction and providing a defined temperature gradient in the axial direction in order to set up a driving force for the movement of the sublimated gas species towards the seed and the growing single crystal.

For the growth of high-quality crystals, homogeneous heat coupling to the growth crucible in the radial direction is considered crucial. By coupling heat as homogeneously as possible, the crystal should also grow as homogeneously and symmetrically as possible. Among other things, this should prevent the formation of threading screw dislocations (TSDs) and threading edge dislocations (TEDs), also called step dislocations, whose formation is favored by an inhomogeneous and asymmetric heat distribution and the associated inhomogeneous and asymmetric growth.

Therefore, every effort is usually made to design a thermally coupled growth system that is homogeneous and ideally radially symmetric, and according to current technology, structural elements that may affect the induced magnetic field are made of materials such as plastic or hard paper or similar composite and structural materials to avoid such effects.

However, the inventors of the present disclosure have discovered that even the most homogeneous, ideally radially symmetrical growth systems that couple heat to the crucible do not allow for the growth of the highest quality crystals, i.e., crystals that are as free of screw and step dislocations as possible.

Published European Patent Application No. 3 699 328 describes a method for improving the quality of SiC single crystals, in particular by using homogeneous insulation.

However, even though the use of homogeneous isolation already favors homogeneous and therefore low-defect growth, the use of optimized isolation is not always sufficient to produce dislocation-free crystals.

Furthermore, published EP 4060098 A1 discloses a method for reducing screw dislocations, in which the screw dislocations present in the seed crystal are made mobile by mechanical tensioning of the seed crystal used, so that, if necessary, the dislocations can annihilate each other when they meet. However, this method does not allow the dislocations newly formed during crystal growth, in particular screw and step dislocations, to be made mobile so that they annihilate each other. As a result, it is not possible to grow crystals that are as free as possible of screw and step dislocations.

U.S. Pat. No. 8,865,324 US Patent Application Publication No. 2016/0138185 US Patent Application Publication No. 2017/0321345 European Patent Application Publication No. 3699328 European Patent Application Publication No. 4060098 European Patent No. 2664695

The present invention has been made in view of the shortcomings and disadvantages of the prior art, and has as its object to provide a system for growing single crystals of semiconductor material by physical vapor transport (PVT) and a method for producing single crystals of semiconductor material in a cost-effective manner with improved single crystal quality.

This object is solved by the subject matter of the independent claims. Advantageous embodiments of the invention are the subject matter of the dependent claims.

The invention is based on the idea that instead of trying to achieve a temperature field during crystal growth that is as uniform and rotationally symmetric as possible, it is also possible to make the growing crystal experience a time-variable temperature field. The inventors of the present disclosure have discovered that such a time variation can move screw and/or step dislocations that may have formed, so that they can encounter and annihilate each other. In particular, since an ideally homogeneous field is clearly not sufficient to obtain the desired crystal quality, the approach presented here is to induce the movement of dislocations during growth by a periodically varying temperature field, so that they cancel each other out.

In particular, a sublimation system for growing at least one single crystal of a semiconductor material by sublimation growth is provided, the sublimation system comprising a crucible having a longitudinal axis and with a fixing means for at least one seed crystal and at least one source material compartment for accommodating source material, a heating system configured to generate an irregular temperature field around the circumference of the crucible and at one or more defined heights along the longitudinal axis of the crucible, and a rotary drive operable to cause a rotational movement of the fixing means about the longitudinal axis relative to the heating system.

Note that a distinction must be made between radial and axial temperature fields.

The radial field, especially in the region of the growing crystal, must have an inhomogeneity that the crystal passes through constantly due to the rotating device. This can occur at different angular ranges and at several heights due to various irregularities in the system.

The axial temperature gradient is the primary driving force for growth, i.e., the transport of sublimated Si and C species can occur because the seed is cooler than the (powdered) source material. However, the axial gradient at the periphery is perturbed by irregularities in the growth system in the same way as the radial gradient.

The two gradients are therefore combined in the radial and axial directions.

Advantageously, the sublimation system has a heating system comprising an induction coil and/or a resistive heating coil operable to generate a magnetic field, in either case the coil at least partially surrounding the crucible. Such coil-type heating systems, whether induction or resistive, can be easily modified to provide an asymmetric temperature field through which the growing single crystal moves.

If the heating system is based on an induction coil, the heating system may comprise an electromagnetic field control element for manipulating the magnetic field. Such an electromagnetic field control element may comprise, for example, a metallic support member and/or a pole piece. Thereby, irregularities can be created in a particularly easy manner without the need to interfere with the coil itself.

On the other hand, the heating system may comprise a coil with a deformed cross section in at least one of its turns and/or with at least one turn arranged to have a different distance from the adjacent turns. In other words, parts of the coil are either deformed in cross section or are axially offset with respect to the remaining equidistant turns. This allows a particularly well-defined scheme for creating an irregular temperature field.

A particularly simple way of introducing irregularities into the temperature field can be achieved if the coil has at least one electrical contact that is not located at the circumferential end of the sublimation system, but rather at an axial position close to the crucible.

To provide the necessary rotation for making the seed crystal and the growing single crystal pass periodically through the asymmetric temperature field, various possibilities exist, which may be used according to the respective structural conditions of the sublimation system. The rotary drive may be coupled to the fixing means such that the seed crystal and the growing single crystal are rotatable relative to the crucible and/or the rotary drive may be coupled to the crucible such that the crucible together with the seed crystal and the growing single crystal is rotatable relative to the heating system.

If the sublimation system includes a thermal insulation element, a rotary drive may be coupled to the thermal insulation element and the crucible, such that the thermal insulation element and the crucible can be rotated relative to the heating system. Any other combination of movable and fixed components can of course be used, as long as the seed crystal moves relative to the radially asymmetric temperature field.

Advantageously, the rotary drive is operable to produce a rotational speed in the range of 1 rpm to 60 rpm, preferably 10 rpm. These speeds may be shown to produce the best results.

The present disclosure further provides a method of growing at least one single crystal of a semiconductor material by sublimation growth, the method comprising:
Providing a crucible having a longitudinal axis, fixing at least one seed crystal to a fixing means of the crucible, and filling at least one source material compartment with source material;
generating a random temperature field around the circumference of the crucible and along the longitudinal axis of the crucible using a heating system;
causing a rotational movement of the fixing means about the longitudinal axis relative to the heating system such that the growing single crystal is exposed to a time-varying temperature field.

As mentioned above, the term "asymmetric" or "irregular" temperature field means that different axial temperature gradients around the circumference correlate with different radial temperature gradients.

According to an advantageous embodiment, a thermal insulator unit is provided between the crucible and the heating system, and the fixing means is rotated relative to the thermal insulator unit.

The crucible may be rotated relative to the heating system and/or relative to a thermal insulation unit provided between the crucible and the heating system.

According to an advantageous embodiment, a thermal insulator unit is provided between the crucible and the heating system, and the crucible and the thermal insulator unit are rotated relative to the heating system.

As mentioned above, the rotational movement may be performed at a rotational speed ranging from 1 rpm to 60 rpm, preferably 10 rpm.

Advantageously, the temperature fields generated have regions that differ from each other by at least 2K and no more than 15K, preferably by 5K.

In particular, the combination of a temperature difference of 5 K and a speed of 10 rpm results in ideal conditions for mobilizing screw and step dislocations.

Through the intentional introduction of asymmetry into the growth system in combination with the rotation device, dislocations, namely TSDs and TEDs, are mobilized and, under appropriate conditions, can cancel each other out when they meet. This reduces the overall dislocation budget and improves the quality of the grown single crystal.

The accompanying drawings are incorporated into and form a part of this specification to illustrate several embodiments of the present invention. These drawings, together with the description, serve to explain the principles of the present invention. The drawings are only for the purpose of illustrating preferred and alternative examples of how the invention can be made and used, and should not be construed as limiting the invention to only the embodiments shown and described. Moreover, several aspects of the embodiments may form solutions in accordance with the present invention, either individually or in different combinations. Thus, the following described embodiments may be considered alone or in any combination thereof. Further features and advantages will become apparent from the following more detailed description of various embodiments of the present invention, as illustrated in the accompanying drawings, in which like reference numerals refer to like elements.

FIG. 1 is a schematic cross-sectional side view of a sublimation system according to a first example. 1 is a schematic cross-sectional side view of a sublimation system according to a further example. 1 is a schematic cross-sectional side view of a sublimation system according to a further example. 1 is a schematic cross-sectional side view of a sublimation system according to a further example. 1 is a schematic top view of a sublimation system according to a further example. 1 is a schematic top view of a sublimation system according to a further example. 1 is a schematic cross-sectional side view of a sublimation system according to a further example. FIG. 1 is a schematic cross-sectional side view of a known sublimation system.

The present invention will now be described in more detail with reference to the figures, first with reference to FIG. 1.

Figure 1 shows a sublimation system 100 according to a first example of the present disclosure. It should be noted that the term sublimation system is intended to encompass any system for growing at least one single crystal of a semiconductor material using a sublimation growth method. Preferably, the term refers to a physical vapor transport (PVT) system for growing silicon carbide (SiC) volume single crystals, as described with reference to Figure 8.

The sublimation system 100 comprises a growth crucible 102, which includes a source material compartment, in particular a SiC supply area 104 and a crystal growth area 106. A powdered SiC source material 108 is injected into the SiC supply area 104 of the growth crucible 102 as a pre-processed starting material before the start of the growth process, e.g., placed in the SiC supply area 104. The source material 108 may be densified or at least partially composed of a solid material in order to increase the density of the source material 108.

A seed crystal 110 is provided in the crystal growth region 106 on an inner wall of the growth crucible 102 facing the SiC supply region 104, for example the crucible lid 112. The bulk SiC single crystal to be grown is grown on the seed crystal 110 by deposition from a SiC growth vapor phase formed in the crystal growth region 106. The bulk SiC single crystal to be grown and the seed crystal 110 may have approximately the same diameter. If the diameter of the crystal channel is larger than the diameter of the seed crystal 110, the bulk SiC single crystal may have a larger diameter than the seed crystal 110. However, the usable low defect diameter of the grown bulk SiC single crystal is typically the same size as the diameter of the seed crystal.

The growth crucible 102, including the crucible lid 112, may be manufactured from an electrically and thermally conductive graphite crucible material. Thermal insulation (not shown in the figures) is disposed around it, which may include, for example, a cellular graphite insulation material, the porosity of which is in particular higher than the porosity of the graphite crucible material.

In the case of induction heating, the thermally insulated growth crucible 102 is placed inside a tubular container (not shown in FIG. 1), which may be configured as a quartz glass tube, forming an autoclave or reactor. An induction heater in the form of a heating coil 116 is placed around the container to heat the growth crucible 102. The heating coil 116 generates the required temperature field by inductively coupling a current into the conductive crucible wall (susceptor) of the growth crucible 102. This current flows as a circulating current in the circumferential direction in the substantially circular, hollow, cylindrical crucible wall, heating the growth crucible 102 in the process. The susceptor can be made of graphite, TaC, WC, Ta, W, or other refractory metals, and can be an integral part of the crucible 102 or a separate part close to the crucible wall. The primary purpose of the susceptor is to provide a heat source inside the crucible 102. When the susceptor is heated by induction, the surface of the susceptor reaches a high temperature, which is then transferred to the inside of the crucible 102 through conduction and/or radiation.

As mentioned above, the coil 116 is mounted on the outside of the glass tube in the case of induction heating and is usually surrounded by a Faraday cage (not visible in FIG. 1) to block electromagnetic radiation. In the case of resistance heating, the coil 116 is also mounted within the reactor and within the thermal insulation, so that it is in close contact with the crucible 102. The principles of the present disclosure are applicable to both heating techniques. Thus, the coil 116 is hereinafter more broadly referred to as a heating means, which encompasses both induction heating and resistance heating (or a combination thereof). Furthermore, it should be noted that in the present disclosure, the coil windings are illustrated as having a round, specifically circular or elliptical cross-section. However, the coil windings may have other suitable cross-sections, such as square or rectangular.

As shown in FIG. 1, the coil 116 has at least one deformation region 115, where the cross section of the coil 116 deviates from the circular cross section of the remaining windings. In FIG. 1, this deviation is depicted as a compression that leads to an elliptical cross section in the deformation region 115. Of course, other cross-sectional irregularities may be applied to the coil 116. In the deformation region 115, the distance between the deformed winding and the adjacent winding is enlarged to a distance d_2, as compared to the normal distance d_1 between the remaining undeformed windings.

In operation, the structural asymmetry caused by this deformation region 115, which may cover, for example, 10° to 90°, preferably 10° to 45°, of the circumference, creates irregularities in the temperature field. These irregularities extend both radially (around the circumference of the crucible) and axially (along the central axis 120).

According to the present disclosure, the sublimation system further comprises a rotational drive (not shown in the figures), which induces a rotational movement of the seed crystal 110, represented by arrow 118, about a central axis 120 while the heating means 116 generates a spatially varying temperature field. The direction of rotation is of course arbitrary.

The rotational motion of the seed crystal together with the growing single crystal in a spatially inhomogeneous temperature field achieves the effect that the seed crystal experiences a time-varying, or in other words fluctuating, temperature field along with the growing single crystal. This dynamic variation of the temperature field acting on the growing crystal moves screw and step dislocations within the growing single crystal such that the dislocations have the opportunity to encounter and annihilate each other.

In other words, the growing crystal undergoes periodic temperature cycling resulting from the combination of a structurally asymmetric growth system with a rotational motion of the growing crystal relative to the temperature field.

It can be shown that in order to mobilize screw and step dislocations, the temperatures measured at two points at a certain height of the growing crystal, at the same radial distance from the rotation axis of the seed crystal 110, must differ by at least 2 K and at most 15 K. Preferably, the temperatures should differ by 5 K.

The axial location of the temperature irregularities may correspond to the current growth/gas boundary or may be anywhere between the seed crystal 110 and the current growth/gas boundary. This means that the periodic temperature cycling acts on the very surface of the growing crystal or in deeper regions to mobilize newly generated or previously generated screw and/or edge dislocations.

The radial distance may correspond to, but is not limited to, the final diameter of the substrate that will be machined from the grown crystal. Also, the diameter can be larger or smaller than the corresponding radius of the resulting SiC substrate.

The growing crystal can also be exposed to a variety of local temperature maxima and minima, since a structurally asymmetric growth system can have multiple asymmetries and irregularities in the resulting temperature field along its longitudinal axis, each having a radial extent of 10° to 90°, preferably 10° to 45°, of the circumference.

For example, for a substrate diameter of 150 mm (i.e., a radius of 75 mm), the magnitude of the temperature irregularity may correspond to a radial spread of 10° to 90°, and thus may be from 1.3 cm up to 11.8 cm, and preferably from 10° to 45°, and thus up to 5.9 cm. These values are calculated using a radius of r=7.5 cm with the following formula for the arc L:
L=[α・π・r]/180
where α is the angle value in degrees.

Therefore, for a diameter of 200 mm, the respective values can be fitted using a radius r = 10.0 cm.

Furthermore, optimal mobilization of screw and step dislocations can be achieved when the seed crystal, together with the growing single crystal, is rotated relative to the heating means at a rotational speed in the range of 1 rpm (revolutions per minute) to 60 rpm, preferably 10 rpm.

Rotation according to the present invention can be provided mechanically by attaching rotational drives to various components of the sublimation system 100, as long as the inhomogeneous temperature field moves relative to the growing single crystal. For example, the fixed part of the seed crystal 110 can be rotated relative to the heating coil 116, with a stationary insulator. Additionally, the heating coil and insulator can be stationary and the crucible 102 can be rotated. Additionally, the crucible and insulator can be rotated relative to the heating coil 116, with the heating coil stationary. Any suitable combination for rotating multiple elements may be applied.

Figure 2 shows another example of how the rotational symmetry of the heating means 116 can be perturbed to create a non-uniform temperature field. Note that the remaining features of the sublimation system 100 correspond to the features of the sublimation system 100 shown in Figure 1.

According to FIG. 2, the heating means comprises a heating coil 116, which has a substantially uniform cross section over all its turns, but has at least one turn that is axially (i.e., along a central axis 120) displaced in a portion of its circumference. Thus, while all remaining turns have a uniform distance d_1 from each other, the displaced region 122 of the coil 116 has a reduced distance d_3 (smaller than d_1) to the near adjacent turn and an increased distance d_2 (larger than d_1) to the far adjacent turn. This intentional asymmetry creates a non-uniform temperature field, which, together with the rotational motion described above with reference to FIG. 1, leads to a dynamic temperature field being experienced by the growing crystal in the crystal growth region 106.

It should be noted that the transition from the normal distance d_1 to the respective maximum displacement (maximum) values d_2 and d_3 along the circumference of the coil 116 may be a gradual transition. However, of course, a step-like transition may be provided by applying respective sharp bends. Again, the coil irregularity may be provided along 10° to 90°, preferably 10° to 45°, of the full 360° circumference of the coil 116.

The rotation is then performed as described above with respect to FIG. 1.

Figure 3 shows another example of how the rotational symmetry of the heating means 116 can be perturbed to create a non-uniform temperature field. Note that the remaining features of the sublimation system 100 correspond to the features of the sublimation system 100 shown in Figure 1.

According to the example shown in FIG. 3, all the windings of the coil 116 are shifted along the central axis 120 with an increased inclination. Thus, two mutually facing regions of each winding are axially offset, for example, by a distance d_1. This results in a rotationally asymmetric temperature field through which the growing single crystal is periodically passed by rotation according to any of the configurations described for the example of FIG. 1.

Furthermore, intentional structural asymmetry can also be introduced into the sublimation system 100 by locating the electrical contacts 124 of the heating coil 116 in the area surrounding the crucible 102. An example of this is shown in FIG. 4. At least one of the electrical contacts 124 that supply current is not located at the circumferential ends of the heating coil 116, but is moved axially to an area where they constitute a structural asymmetry near the crucible 102, particularly near the crystal growth region 106. Thus, during operation, the heating coil 116 generates a non-uniform temperature field through which the growing single crystal rotates. This rotation leads to a pulsed temperature field in the growing single crystal, mobilizing step and screw dislocations. The moving step and screw dislocations encounter each other and annihilate each other, which can significantly improve the overall quality of the grown single crystal boule.

Figure 5 shows a top view of a sublimation system 100 according to a further example of the present disclosure. In this view, the heating coil 116 is disposed inside a shield 126 that shields the external environment from electromagnetic radiation. The coil 116 surrounds a reactor 114 in which the seed crystal 110 resides.

According to this advantageous example, one or more first pole pieces 128 (also called pole pieces) are arranged at one or more different positions (and axial heights) along the windings of the coil. These pole pieces 128 act as inductive elements of the electromagnetic field generated by the coil 116. Thus, an intentional asymmetry is introduced in the resulting temperature field. As mentioned with reference to the previous example, the sublimation system 100 further comprises a rotational drive (not shown in the figures), which causes a rotational movement of the seed crystal 110, represented by the arrow 118, around the central axis 120 while the heating means 116 generate a spatially varying temperature field. The direction of rotation is of course arbitrary.

The rotational motion 118 of the seed crystal 110 together with the growing single crystal in the spatially inhomogeneous temperature field achieves the effect that the seed crystal 110 along with the growing single crystal experiences a time-varying, or in other words, fluctuating, temperature field. This dynamic fluctuation moves the screw and step dislocations within the growing single crystal such that the dislocations have the opportunity to encounter and annihilate each other.

In other words, the growing crystal undergoes periodic temperature cycling resulting from the combination of a structurally asymmetric growth system with a rotational motion of the growing crystal relative to the temperature field.

Rotation according to the present disclosure may be provided mechanically by attaching rotational drives to the various components, so long as the inhomogeneous temperature field moves relative to the growing single crystal. For example, the fixed part of the seed crystal 110 may be rotated relative to the heating coil 116, with a stationary insulator. Additionally, the crucible 102 may be rotated, with the heating coil and insulator stationary. Additionally, the crucible and insulator may be rotated relative to the heating coil 116, with the heating coil stationary. Any suitable combination for rotating multiple elements may be applied.

Additionally or alternatively, a second pole piece 130 may be provided on the electromagnetic shield 126 and extend into the gap between the shield 126 and the coil 116. Again, this pole piece 130 creates a structural asymmetry near the crucible 102, particularly near the crystal growth region 106. Thus, during operation, the heating coil 116 creates a non-uniform temperature field through which the growing single crystal rotates. This rotation leads to a pulsed temperature field in the growing single crystal, mobilizing step and screw dislocations. The moving step and screw dislocations may encounter and annihilate each other, thereby significantly improving the overall quality of the grown single crystal boule.

6 shows a top view of a sublimation system 100 according to a further advantageous example. As shown in this figure, the asymmetry of the temperature field may be created by an asymmetric shield 126 and a metallic holder 132 for the shield 126. The holder 132 may advantageously extend into the gap between the coil 116 and the shield 126.

Thus, during operation, the heating coil 116 creates a non-uniform temperature field through which the growing single crystal is rotated, as indicated by arrow 118. Thus, screw and step dislocations in the growing single crystal are mobilized so that they can encounter and annihilate each other.

Turning now to FIG. 7, the principles of the present disclosure may further be applied to a sublimation system 200 operable to grow two single crystal boules simultaneously. To this end, a crucible 202 is provided with a first seed crystal 210A and a second seed crystal 210B. FIG. 7 shows an example similar to the configuration shown in FIG. 4. However, it is clear that any other possibility of generating an asymmetric temperature field, as described for example in EP 2 664 695, may be applied to the sublimation system 200 for growing two or more single crystal boules simultaneously.

In particular, FIG. 7 shows a schematic cross-sectional view of a physical vapor transport (PVT) growth system 200 for simultaneously growing two SiC bulk crystals. The system 200 comprises a crucible 202, which includes a central source compartment 234 containing SiC powder 208, a SiC source material.

Two seed crystals 210A and 210B are placed in the growth regions 206A and 206B. Each of the growth regions 206A and 206B is separated from the powdered, compressed, or solid SiC source material 208 by a gas-permeable porous barrier 236A, 236B, thus ensuring that only gaseous Si- and C-containing components enter the growth regions 206A and 206B. A heating coil 216 provides the necessary temperature field. The asymmetry of the temperature field is achieved by providing electrical contacts 224 in the vicinity of the crystal growth regions 206A, 206B.

Both seed crystals 210A, 210B are rotated in an asymmetric temperature field generated by coil 216 to mobilize step and/or screw dislocations in the growing single crystal, as described above with reference to Figures 1-6.

In summary, the present disclosure is based on the idea that instead of trying to achieve a temperature field that is as uniform and rotationally symmetric as possible during crystal growth, it is also possible to have the growing crystal experience a time-variable temperature field. The inventors of the present disclosure have discovered that such a time variation can move any formed screw and/or step dislocations, so that they encounter and annihilate each other. In particular, since an ideally homogeneous field is clearly not sufficient to obtain the desired crystal quality, the approach presented here is to induce the movement of dislocations during growth by a periodically varying temperature field, so that the dislocations cancel each other out.

For this purpose, several conditions must be met. The solution to the technical problem of growing crystals that are nearly free of screw and step dislocations lies on the one hand in the design of a growth system that deliberately creates an inhomogeneous and radially asymmetric thermal coupling to the growth crucible.

Such non-homogeneous and asymmetric thermal coupling may be achieved, for example, by the following features of the crystal growth system:
Non-equidistant spacing of the coil turns due to uneven alignment Non-equidistant spacing of the coil turns due to uneven configuration of the turns Inlets and outlets to the coil/resistance heater in the area of the susceptor and growth crucible Ends of the coil turns (in the area of the susceptor and growth crucible) Insertion of one or more induction field guides, e.g. at least one metallic post, pole piece, etc.; the conductive components can be attached directly to the induction coil or between the induction coil and the electromagnetic shield Use of asymmetric shielding of the electromagnetic field

On the other hand, a solution to the technical problem of growing crystals that are nearly free of screw and/or step dislocations consists in installing a rotating device within the growth system that is inhomogeneously thermally coupled.

Rotation can induce dislocation dynamics in the following configurations:
Rotating seed with stationary insulator/coil/resistance heating Rotating crucible with stationary insulator/coil/resistance heating Rotating crucible and insulator with stationary coil/resistance heating Combinations for rotating multiple elements

It is through the combination of the rotating device and the asymmetric growth system according to the present invention that a dynamic method of mobilizing screw and step dislocations is implemented. This mobilization allows the dislocations to meet and cancel each other.

The growing crystal rotates through cold and hot areas in a repetitive (sinusoidal) manner due to the special design of the crystal growth system and the resulting asymmetric thermal coupling.

To mobilize dislocations, the temperatures measured at two points on the diameter of a substrate made from the crystal must differ by at least 2 K, and at most 15 K, and preferably 5 K. The rotation frequency should be between 1 and 60 revolutions per minute (rpm), preferably 10 rpm, for optimal mobilization of dislocations. This combination of a 5 K temperature difference and 10 rpm results in ideal conditions for mobilizing screw and step dislocations.

Through the intentional introduction of asymmetry into the growth system in combination with the rotation device, dislocations, namely TSDs and TEDs, are mobilized and, under appropriate conditions, can cancel each other out when they meet. This reduces the overall dislocation budget and improves the quality of the grown single crystal.

100, 200 Sublimation system, PVT system 102, 202 Crucible 104 SiC supply area, source material compartment 106, 206A, 206B Crystal growth area 108, 208 Source material 110, 210A, 210B Seed crystal 112 Crucible lid 114 Container, reactor 115 Deformation area 116, 216 Induction or resistance heating coil, heating means 118, 218 Rotational movement 120, 220 Central axis 122 Displacement area 124, 224 Electrical contacts 126 Shielding 128 First pole piece 130 Second pole piece 132 Holder 234 Source material compartment 236A, 236B Barrier 800 PVT system 802 Crucible 804 SiC supply area 806 Crystal growth area 808 Raw material 810 Seed crystal 812 Crucible lid 814 Container, reactor 816 Induction heating coil

Claims (15)

A sublimation system for growing at least one single crystal of a semiconductor material by sublimation growth, the sublimation system (100) comprising:
A crucible (102) having a longitudinal axis (120) and comprising fixing means for at least one seed crystal (110) and at least one source material compartment (104) for receiving source material (108);
a heating system configured to generate a random temperature field around a circumference of the crucible at one or more defined heights along the longitudinal axis of the crucible;
a rotational drive operable to cause rotational movement of said fastening means about said longitudinal axis relative to said heating system. The sublimation system of claim 1, wherein the heating system comprises an induction coil (116) and/or a resistive heating coil (116) operable to generate an electromagnetic field, the induction coil (116) and/or the resistive heating coil (116) at least partially surrounding the crucible (102). The sublimation system of claim 2, wherein the heating system includes an electromagnetic field control element for manipulating the electromagnetic field. The sublimation system of claim 3, wherein the electromagnetic field control elements comprise metallic support members and/or pole pieces. The sublimation system of any one of claims 2 to 4, wherein the coil (116) has a deformed cross-section in at least one of its windings and/or the coil (116) has at least one winding arranged to have a different distance from adjacent windings. The sublimation system of any one of claims 2 to 5, wherein the coil (116) comprises at least one electrical contact (124) disposed in an axial position adjacent to the crucible (102). The sublimation system according to any one of claims 1 to 6, wherein the rotary drive is coupled to the fixing means such that the seed crystal (110) is rotatable relative to the crucible (102) and/or the rotary drive is coupled to the crucible (102) such that the crucible (102) is rotatable relative to the heating system. The sublimation system of any one of claims 1 to 7, wherein the sublimation system (100) comprises a thermal insulation element, and the rotary drive is coupled to the thermal insulation element and the crucible (102), such that the thermal insulation element and the crucible (102) are rotatable relative to the heating system. A sublimation system according to any one of claims 1 to 8, wherein the rotary drive is operable to produce a rotational speed in the range of 1 rpm to 60 rpm, preferably 10 rpm. 1. A method for growing at least one single crystal of a semiconductor material by sublimation growth, comprising the steps of:
Providing a crucible (102) having a longitudinal axis, fixing at least one seed crystal (110) to a fixing means of said crucible, and filling at least one source material compartment (104) with source material (108);
generating a random temperature field around the circumference of the crucible (102) and along the longitudinal axis of the crucible (102) using a heating system;
and causing a rotational movement of said fixing means about said longitudinal axis relative to said heating system such that said growing single crystal is exposed to a time-varying temperature field. The method of claim 10, wherein a thermal insulator unit is provided between the crucible (102) and the heating system, and the fixing means is rotated relative to the thermal insulator unit. The method of claim 10 or 11, wherein the crucible is rotated relative to the heating system and/or relative to a thermal insulation unit provided between the crucible and the heating system. The method according to any one of claims 10 to 12, wherein a thermal insulator unit is provided between the crucible and the heating system, and the crucible and the thermal insulator unit are rotated relative to the heating system. The method according to any one of claims 10 to 13, wherein the rotational movement is performed at a rotational speed in the range of 1 rpm to 60 rpm, preferably 10 rpm. 15. The method according to one of claims 10 to 14, wherein the generated temperature fields have regions which differ from each other by at least 2K and up to 15K, preferably by 5K.